The state of the art discloses normally open or normally closed solenoid valves, which are used as inlet valves or outlet valves in a hydraulic unit of a vehicle braking system, for example. The hydraulic unit serves for performing open and/or closed-loop control operations in an antilock braking system (ABS) or a traction control system (TCS) or an electronic stability program system (ESP) in order to build up or reduce pressure in corresponding wheel brake calipers. Such solenoid valves comprise a solenoid assembly and a valve cartridge, which comprises a pole body, a guide sleeve connected to the pole body, a valve armature having a closing element and axially moveable and guided inside the guide sleeve against the force of a return spring between a closed position and an open position, and a valve sleeve connected to the guide sleeve and having a valve seat. The valve seat is arranged between at least one first flow port and at least one second flow port. The closing element in the closed position has a sealing interaction with the valve seat and interrupts a fluid flow between at least the one first flow port and at least the one second flow port. In the open position the closing element is lifted off from the valve seat and allows the fluid flow between at least the one first flow port and at least the one second flow port. Energizing of the solenoid assembly generates a magnetic force, which in an unenergized, open solenoid valve moves the valve armature with the closing element from the open position into the closed position until the closing element meets the corresponding valve seat and seals the latter. In the unenergized state the return spring moves the valve armature with the closing element and the closing element lifts off from the valve seat and opens the latter. In an unenergized, closed solenoid valve the valve armature with the closing element is moved from the closed position into the open position by the energizing of the solenoid assembly and the closing element lifts off from the valve seat and opens the latter. If the current is switched off, the return spring moves the solenoid armature with the closing element in the direction of the valve seat until the closing element meets the valve seat and seals the latter. The solenoid valves described produce a so-called closing noise when the closing element, which comprises a hardened closing member that is embodied as a steel ball, for example, meets the valve seat likewise composed of hardened steel.
The published patent application DE 10 2007 051 557 A1, for example, describes a normally closed solenoid valve for a vehicle hydraulic braking system with wheel-slip control. The solenoid valve comprises a hydraulic part, also referred to as a valve cartridge, which is partially arranged in a stepped bore of a valve block, and an electrical part, which is substantially formed from a solenoid assembly, which is fitted onto the part of the valve cartridge projecting from the valve block. The solenoid assembly comprises a coil shell with an electrical winding, a coil casing that carries magnetic flux, and an annular disk that carries magnetic flux. The hydraulic part comprises a guide sleeve, which at its end facing the electrical part is closed by a pole body that is pressed in and welded so that it is fluid-tight. A longitudinally displaceable armature, which is supported by a return spring on the pole body, is accommodated in the guide sleeve. Remote from the pole body, the armature comprises a spherical closing member arranged in a depression. At the end remote from the pole body, a cupped valve sleeve having a cylindrical shell and a base is pressed into the guide sleeve. The valve sleeve has a passage on the base together with a hollow cone-shaped valve seat, which with the closing member forms a seat valve. The seat valve forms a controllable fluid connection between the passage on the base of the valve sleeve and at least one passage in the shell of the valve sleeve. A radial filter is moreover arranged externally on the shell of the valve sleeve, in order to filter dirt particles out of the fluid flow. The guide sleeve may be calked in the stepped bore of the valve block by means of a fixing bush.